The present invention disclosed herein relates to quantum cryptography technologies, and more particularly, to polarization coding quantum cryptography systems.
The security problems on communication networks are recently emerging as very important issues, with the rapid development of wired/wireless communication technologies and the widespread use of various communication services. The securities of communication networks are becoming increasingly important, particularly in terms of the protection of secrets and personal information related to nations, enterprises and finances. A quantum cryptography technology is recently attracting much attention as a solution to the security problems on communication networks. The quantum cryptography technology is a communication security technology that guarantees the security by the principle of quantum mechanics, one of the natural basic laws, thus making eavesdrop or interception absolutely impossible. The quantum cryptography technology is also known as a quantum key distribution (QKD) technology that is based on the laws of quantum physics such as a no-cloning theorem to distribute, in an absolutely secure way, secret keys that are used to encrypt/decrypt data transmitted between a transmitter and a receiver.
Typical quantum cryptography or quantum key distribution methods are described in detail in the review paper [N. Gisin, G. Ribordy, W. Tittel and H. Zbinden, “Quantum Cryptography”, Rev. Mod. Phys. vol. 74, pp. 145-195 (2002)]. According to this paper, examples of the typical quantum cryptography or quantum key distribution methods include BB84, B92, and EPR protocols.
A quantum cryptography key distribution method, known as the BB84 protocol, is disclosed in the paper [Charles Bennett and Gilles Brassard, “Quantum Cryptography: Public key distribution and coin tossing”, Proc. IEEE Int. Conf. on Computers, Systems and Signal Processing, Bangalore, India, pp. 175-179 (IEEE, New York, 1984)]. The quantum cryptography key distribution method uses four quantum states constituting two bases (e.g., photon polarization states 0°, 90°, 45° and 135°).
Specifically, a transmitter unit (Alice) randomly selects one of the two bases, randomly selects one of the two quantum states of the selected basis (a bit value of a secret key), i.e., one of ‘0’ and ‘1’, and transmits the selected one through a quantum channel to a receiver unit (Bob). For example, if polarization states of a single photon, i.e., two bases of (0°, 90°) and (45°, 135°) are used, in which 0° and 45° represent a bit value ‘0’ and 90° and 135° represent a bit value ‘1’, and if the (0°, 90°)-basis and the bit value ‘1’ are randomly selected by the transmitter unit, the transmitter unit transmits a single photon with a polarization state of 90° through a quantum channel to the receiver unit.
Upon receiving the single photon from the transmitter unit, the receiver unit randomly selects one of two bases and uses the selected basis to measure the quantum state of the received single photon. After the measurement by the receiver unit, the transmitter/receiver units reveal their selected bases through a classical channel to each other. Herein, if the basis selected by the transmitter unit is identical to the basis selected by the receiver unit, because the measurement result of the receiver unit accords with the quantum state selected by the transmitter unit, the transmitter/receiver units come to have the same bit value.
A bit string, which is generated by repeating the above process and sifting only bit values corresponding to the case of the same basis being selected by the transmitter/receiver units, is called a sifted key. The sifted key is processed through post-processing processes such as error correction and privacy amplification, and the processed sifted key is used as a secret key.
If an eavesdropper (Eve) attempts to eavesdrop in the process, an error occurs in a sifted key obtained by the transmitter/receiver units by the basic principle of quantum mechanics. The transmitter/receiver units reveal a portion of the sifted key to each other and calculate an error rate to determine if there is an eavesdropper.
Meanwhile, it is difficult to implement an ideal single-photon light source. Therefore, four light sources generating photons of different polarization states are used to implement the BB84 protocol requiring four polarization states. Similarly, two light sources generating photons of different polarization states are used to implement the B92 protocol requiring two polarization states (for example, see [“A Short Wavelength GigaHertz Clocked Fiber-Optic Quantum Key Distribution System”, IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 40, NO. 7, JULY 2004]). However, spectrums of light sources may differ from each other because multiple light sources cannot be implemented in exactly the same way as a single light source. An eavesdropper can attempt to eavesdrop using such a spectrum difference. Therefore, a quantum cryptography system using multiple light sources is inferior in security to an ideal quantum cryptography system using a single photon.
Methods for constructing a quantum cryptography system by use of a single light source and a polarization modulator have been proposed to overcome the above technical limitation. These methods, however, are reported to be difficult to achieve stable operation characteristics.